1. Industrial Field of the Invention
The present invention relates to a surface mount antenna and a communication apparatus including the same.
2. Description of the Related Art
FIG. 4 shows a conventional surface mount antenna. The basic configuration of the surface antenna is stated in Japanese Unexamined Patent Publication No. 9-098015. In the surface mount antenna 1 shown in FIG. 4, a ground electrode 3 is provided on a first major surface 2a of a substrate 2 made of a dielectric such as a ceramic, a resin, and the like, having a substantially rectangular prism shape. A strip radiation electrode 4 is provided on a second surface 2b. A feeding electrode 5 is provided on the first major surface 2a, turning to and elongating on one end surface of the substrate 2. The radiation electrode 4 is provided on substantially the entire surface of the second major surface 2b of the substrate 2, in a straight strip shape. In one end of the radiation electrode 4, an open end portion 4a is so provided as to extend and turn to the end surface in which the feeding electrode 5 is provided, till it is close to the feeding electrode 5 through a gap 6. In the other end of the radiation electrode 4, a grounded end portion 4b is so provided as to extend and turn to and on the end surface of the substrate 2 in which the feeding electrode 5 is provided, till it gets into contact with the ground electrode 3.
With the above configuration of the surface mount antenna 1, a high frequency signal, inputted into the feeding electrode 5, is transmitted to the radiation electrode 4 through a static capacitance produced in the gap 6 between the open end portion 4a of the radiation electrode 4 and the feeding electrode 5. The radiation electrode 4 of which one end is open and the other end is short-circuited, having a length of about .lambda./4 becomes resonant as a microstrip line resonator. With the resonance, a part of an electric field energy generated between the radiation electrode 4 and the ground electrode 3 is radiated into space. Thus, the surface mount antenna 1 acts as an antenna. In FIG. 4. J1 designates an electric current flowing in the radiation electrode 4.
In a microstrip line resonator with a length of about .lambda./4 such as the surface mount antenna 1, a current flowing there is maximum in the grounded end portion 4b. The grounded end portion 4b is so provided as to be narrow in width, as compared with the other part of the radiation electrode 4. Therefore, the current density in the grounded end portion 4b is higher than the other part of the radiation electrode 4. Accordingly, the conductor loss of the radiation electrode 4 is maximum in the grounded end portion 4b.
FIG. 5 is a side view showing the end surface of the surface mount antenna 1 where the feeding electrode 5, the open end portion 4a of the radiation electrode 4, and the grounded end portion 4b are provided. As shown in FIG. 5, the current J1 flows in the grounded end portion 4b of the radiation electrode 4, concentratedly in the side of the grounded end portion 4b which is relatively near to the open end portion 4a, since the current J1 has a tendency to flow from the feeding electrode 5 to the ground electrode 3 a shortest distance, due to its electromagnetic characteristics.
As described above, the maximum current flowing in the radiation electrode 4 is concentrated in the side of the narrow grounded end portion 4b which is relatively near to the open end portion 4a. Therefore, the conductor loss in the above-mentioned side of the grounded end portion 4b is high, which causes the antenna gain to decrease.
It is thinkable that the width of the grounded end portion 4b is widened as measures for reducing the conductor loss. Even if the grounded end portion 4b is widened, a current flows concentratedly in the side of the grounded end portion 4b which is relatively close to the open end portion 4a. Accordingly, the antenna gain can not be improved. By widening the grounded end portion 4b of the radiation electrode 4, the distance between the grounded end portion 4b and the open end portion 4a becomes short. That is, the substantial length of the radiation electrode is shortened, causing the resonance frequency to increase. In order to restore the resonance frequency, it is necessary to lengthening the substrate 2 and the radiation electrode 4.